Allow nil middleware to be returned

In case a middleware actually just wants some code to execute at startup... will expand on that idea later.

config/dispenser.go

@@ -8,12 +8,14 @@ import (
 // dispenser is a type that gets exposed to middleware
 // generators so that they can parse tokens to configure
 // their instance.
+// TODO: Rename this; it does more than dispense tokens.
+// It also provides access to the server to touch its
+// configuration.
 type dispenser struct {
 	parser  *parser
 	cursor  int
 	nesting int
 	tokens  []token
-	//err     error
 }
 
 // newDispenser returns a new dispenser.

config/parser.go

@@ -113,7 +113,9 @@ func (p *parser) unwrap() error {
 				if err != nil {
 					return err
 				}
-				p.cfg.Middleware = append(p.cfg.Middleware, mid)
+				if mid != nil {
+					p.cfg.Middleware = append(p.cfg.Middleware, mid)
+				}
 			} else {
 				return errors.New("No middleware bound to directive '" + directive + "'")
 			}